The present invention is generally directed to a processes for the preparation of polymeric particulate materials with narrow molecular weight distribution properties and narrow particle size distribution properties. More specifically, the present invention relates to improved reliability in polymerization processes for obtaining stable latexes (latices), and improved control over the resulting latex particle size and particle size distribution.
The present invention relates to processes for the preparation of polymers, and more specifically to miniemulsion polymerization processes, and to polymeric resins formed thereby. In one embodiment, the present invention relates to a stable free radical moderated miniemulsion polymerization process for producing a thermoplastic polymer resin or resins, that have narrow polydispersities, that is, narrow molecular weight distributions as defined by the ratio Mw:Mn, where Mw is weight average molecular weight of the polymer and Mn is number average molecular weight of the polymer, with an easily controllable modality, from at least one monomer compound comprising heating for an effective period of time a miniemulsified mixture of water, a free radical initiator, a stable free radical agent, an emulsifier, an optional co-surfactant and at least one substantially water insoluble polymerizable monomer compound under conditions such that all polymer chains are initiated at about the same time; cooling the mixture to effectively terminate the polymerization; optionally isolating the thermoplastic resin product; and optionally washing and drying the polymer resin products. Related miniemulsion polymerization processes for preparing, for example, thermoplastic resin are accomplished, in other embodiments, comprising: forming a miniemulsion comprised of a mixture of an oligomeric compound of the formula R-SFR, wherein R is an oligomeric compound comprised of from about 1 to about 30 monomer units and -SFR is a covalently bound stable free radical end group, at least one free radical polymerizable monomer compound, and a surfactant; and heating the miniemulsion from, for example, about 70 to about 200.degree. C. for from, for example, about 1 to about 40 hours, preferably in a sealed vessel, wherein there results a latex thermoplastic resin or a latex of thermoplastic resins with a high monomer to polymer conversion of about 85 to 100 percent and a narrow polydispersity of from about 1.1 to about 2.0.
In another embodiment polymeric chain growth proceeds by a pseudoliving mechanism and can provide resins of variable molecular weights from very low to very high, for example, less than about 10,000 up to about 200,000 while maintaining narrow molecular weight distributions or polydispersities. In another embodiment block and multiblock copolymers can be synthesized by the aforementioned stable free radical moderated emulsion polymerization processes wherein each block formed is well defined in length by the added and reacted monomer and wherein each additional block that is formed also possesses a narrow molecular weight distribution.
Emulsion polymerization processes are known in the art, as illustrated hereinafter. Conventional emulsion polymerization processes proceed by a free radical mechanism providing resins of broad polydispersities and generally high molecular weights. The present invention relates to a miniemulsion polymerization process that proceeds via a pseudoliving free radical mechanism and provides resins of high, intermediate, or low molecular weights and with narrow polydispersities. The present invention provides product resins with a latent thermally reactive functional group on at least one end which can be used for further reaction to prepare other resins with complex architectures. The present invention, in embodiments, provides for control of the miniemulsion droplet size and the resultant resin bead or particle size, for example, as disclosed in the aforementioned copending application now U.S. Pat. No. 5,852,140. The aforementioned commonly owned U.S. Ser. No. 08/214,518, discloses an emulsion polymerization process for the preparation of a thermoplastic resin or resins comprising heating from about 70 to about 160.degree. C. a mixture comprised of water or aqueous mixtures of polar protic solvents, a water soluble or water insoluble free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound which is substantially insoluble in water to form a latex thermoplastic resin or a latex of thermoplastic resins with a high monomer to polymer conversion and a narrow polydispersity.
Emulsion polymerization processes are industrially important, and are the predominant process for the synthesis of a number of copolymers, such as for example styrene/butadiene. However, resins prepared by emulsion polymerization processes typically have broad polydispersities and high molecular weights. When low molecular weight resins are required, a chain transfer agent is typically added to limit the extent of chain growth by way of premature chain termination events and which agent is, for example, an unpleasant smelling thiol compound. Polymers prepared by thiol type chain transfer mediated emulsion polymerization processes are terminated at either end with functional groups which preclude further free radical reactions and therefore limits the utility of the polymer resin products produced therefrom.
The present invention is directed to pseudoliving miniemulsion polymerization processes which permit the economic preparation of narrow polydispersity resins with low, intermediate, or high molecular weights. The low molecular weight resins can be prepared without a chain transfer agent or molecular weight modifier which provides several advantages over conventional emulsion polymerization processes as illustrated herein.
The stable free radical mediated miniemulsion polymerization system of the present invention can readily afford narrow polydispersities, in embodiments, of from about 2.0 to about 3.0, and in embodiments, of from about 1.15 to about 1.25 for polystyrene, and as low as 1.5 or below for various copolymer systems. Stable free radical mediated miniemulsion polymerization systems of the present invention afford resin polydispersities that are comparable to those obtained in anionic polymerizations.